TECHNICAL FIELD
[0001] The present invention relates to the technical field of mobile communication, and
more particularly relates to a base station apparatus and a method for use in a mobile
communication system.
BACKGROUND ART
[0002] In this technical field, a LTE (Long Term Evolution) scheme is being discussed in
a W-CDMA standardization organization 3GPP as a successor communication scheme of
a W-CDMA (Wideband-Code Division Multiple Access) scheme, a HSDPA (High Speed Downlink
Packet Access) scheme and a HSUPA (High Speed Uplink Packet Access) scheme. In the
LTE scheme, an inactive (idle) user apparatus may migrate between cells while performing
DRX (Discontinuous Reception) and is managed for each TA (Tracking Area) including
one or more cells while maintaining the latest location registration state. On the
other hand, a base station conducts DTX (Discontinuous Transmission) in downlinks
for that user apparatus so that the user apparatus can conduct the DRX properly. In
accordance with the LTE scheme, an active user apparatus may also conduct the DRX
as needed mainly in terms of battery energy savings.
[0003] A user apparatus conducting the DRX switches between an active state and an inactive
state in fixed cycles (DRX cycle) and receives L1/L2 control signals at the cycles.
The user apparatus demodulates the L1/L2 control signals and determines whether there
is information destined for the user apparatus. The information may include the presence
of downlink data, a resource block and a data format such as a data modulation scheme
to be used if the downlink data is present, and a resource block and a data format
available for the next uplink data transmission. If information destined for the user
apparatus is present, the user apparatus may receive downlink data in accordance with
the information. Otherwise, the user apparatus may transition from the active state
to the inactive state and wait for the next activation timing. In general, a longer
DRX cycle leads to greater effect on the battery savings or power consumption. However,
it should be noted that the DRX may affect QoS (Quality of Service). Some techniques
for setting the DRX cycle for each radio bearer are disclosed in
3GPP R2-070463, February 6, 2007, for example.
[0004] Many user apparatuses conducting the DRX can be arranged to activate in different
subframes.
[0005] FIG. 1 illustrates ten types of activation timing patterns (patterns 1-10) for a
certain DRX cycle. User apparatuses conduct the DRX in accordance with any of these
patterns. For any pattern, the user apparatus is active during one TTI and inactive
during nine TTIs. The many user apparatuses are arranged not to concentrate on a certain
pattern. For example, if the user apparatuses are associated with patterns 1-10, patterns
1, 2, ..., 10 may be cyclically assigned to the user apparatuses. Alternatively, any
of patterns 1-10 may be randomly used. Alternatively, a remainder of division of an
identification number of a user apparatus by 10, for example, C-RNTI mod 10, may be
used as the pattern number. Even in any of the cases, the user apparatuses are distributed
over all patterns 1-10.
[0006] A limited amount of resources can be used for L1/L2 control channels and data signals,
and accordingly a limited number of user apparatuses are allowed (scheduled for) transmissions
in a single radio frame. As a result, as illustrated in the upper side of FIG. 2,
if a smaller number of user apparatuses belong to a certain pattern, there would be
a higher likelihood that resource blocks may be assigned for transmissions of downlink
data (shared data channels). On the other hand, as illustrated in the lower side of
FIG. 2, if a larger number of user apparatuses belong to a certain pattern, there
would be a lower likelihood that resource blocks may be assigned for transmissions
of downlink data (shared data channels). If data and control signals desired to be
transmitted to a user apparatus at a DRX activation timing are not scheduled, the
transmission of the data and control signals destined for the user apparatus would
be delayed to the next activation timing.
[0007] On the other hand, the user apparatuses ubiquitously reside within a service area
and are in various communication environments. Thus, even if the user apparatuses
receive the same radio bearer, the user apparatuses may have different channel states
and different radio transmission states.
[0008] There is a high risk that a user apparatus with a poor channel state, such as a user
apparatus residing in a cell boundary, may fail to receive a downlink L1/L2 control
signal and a subsequent data signal. If the user apparatus fails to receive the downlink
signals, the user apparatus cannot be communicating until the next activation timing,
resulting in longer transmission delay of the downlink signals. This may be more significant
for a longer DRX cycle. Particularly, the user apparatus residing in a cell boundary
may fail to receive the downlink signals with a high likelihood compared to a user
apparatus residing near a base station. When the user apparatus residing in a cell
boundary is communicating information associated with handover control, it is undesirable
to make the above transmission delay longer.
DISCLOSURE OF INVENTION
[PROBLEM TO BE SOLVED BY THE INVENTION]
[0009] In a conventional scheme, user apparatuses are assigned to DRX patterns 1-10 in such
a manner that the user apparatuses can be evenly distributed over the DRX patterns
1-10. In this case, however, there is a risk that relatively longer delay may arise
for user apparatuses with a relatively poor channel state.
[0010] Thus, one object of the present invention is to shorten the delay that may arise
for user apparatuses with a relatively poor channel state among user apparatuses conducting
the DRX.
[MEANS FOR SOLVING THE PROBLEM]
[0011] One aspect of the present invention relates to a base station apparatus, including:
a classification unit configured to classify user apparatuses based on channel state
of a radio link; a DRX (Discontinuous Reception) status memory configured to store
information indicating which of multiple groups each of one or more of the user apparatuses
conducting DRX belongs to; a determination unit configured to determine which of the
groups the user apparatuses classified by the classification unit are to belong to
with reference to the information stored in the DRX status memory; and an indication
unit configured to indicate the determined groups to the user apparatuses, wherein
each of the groups includes one or more of the user apparatuses activating simultaneously,
and the determination unit is configured to assign one of the user apparatuses having
relatively poor channel state to one of the groups having a smaller number of simultaneously
activating user apparatuses.
[ADVANTAGE OF THE INVENTION]
[0012] According to the present invention, it is possible to shorten the delay that may
arise for user apparatuses with a relatively poor channel state among user apparatuses
conducting the DRX.
BRIEF DESCRIPTION OF DRAWINGS
[0013]
FIG. 1 schematically illustrates various DRX timing patterns;
FIG. 2 illustrates likelihoods of resource assignment corresponding to different numbers
of user apparatuses belonging to a certain activation pattern;
FIG. 3 is a functional block diagram illustrating a base station according to one
embodiment of the present invention;
FIG. 4 schematically illustrates exemplary contents stored in an assignment status
memory; and
FIG. 5 is a flowchart illustrating an exemplary operation of a base station according
to one embodiment of the present invention.
LIST OF REFERENCE SYMBOLS
[0014]
31: radio receiving unit (RxRF)
32: channel state evaluation unit
33: determination unit
34: DRX group determination unit
35: assignment status memory
36: indicating signal generation unit
37: radio transmitting unit (TxRF)
BEST MODE FOR CARRYING OUT THE INVENTION
[0015] According to one aspect of the present invention, a user apparatus having poor channel
state (user apparatus residing in cell boundary) is assigned to a group having a smaller
number of simultaneously activating user apparatuses. When the user apparatus is assigned
to the group having the smaller number of user apparatuses, the user apparatus could
have more opportunities to have radio resources assigned in that group. In this manner,
the user apparatus having poor channel state is assigned to the group, which can shorten
delay that may arise for the user apparatus.
[0016] In addition, the user apparatus may be assigned to a group having a smaller number
of user apparatuses having poor channel state rather than a smaller total number of
user apparatuses. In general, L1/L2 control signals for the user apparatus having
a poor channel state may require a larger amount of radio resources than the amount
for a user apparatus having a satisfactory channel state. This may arise from utilization
of a MCS having a relatively low coding rate. As a result, if a larger number of user
apparatuses having poor channel states belong to the same group, a larger amount of
radio resources may be used for the L1/L2 control signals for that group, resulting
in reduction in the corresponding amount of radio resources available for shared data
channels. In this manner, a single group is arranged to include a smaller number of
user apparatuses having poor channel states. As a result, a smaller amount of radio
resources can be assigned for the L1/L2 control signals used for the group, which
can reserve for a larger amount of radio resources for the shared data channels.
[0017] As stated above, there is a higher risk that a user apparatus having poor channel
state may fail to receive downlink signals compared to a user apparatus having satisfactory
channel state. However, the user apparatus having the poor channel state could obtain
more opportunities to have radio resources assigned through the assignment of a DRX
group having a smaller number of user apparatuses to that user apparatus. As a result,
it can be expected that the user may experience similar delay independently of channel
quality or at least a difference between delays associated with the poor channel state
and the satisfactory channel state may be reduced. Thus, the present invention can
contribute to uniform service among different areas within a service area.
[0018] For convenience, specific values may be used. However, the specific values are simply
illustrative, and other values may be used as needed.
[First embodiment]
[0019] FIG. 3 is a functional block diagram illustrating a base station according to one
embodiment of the present invention. In FIG. 3, a radio receiving unit (RxRF) 31,
a channel state evaluation unit 32, a determination unit 33, a DRX group determination
unit 34, an assignment status memory 35, an indicating signal generation unit 36 and
a radio transmitting unit (TxRF) 37 are illustrated.
[0020] The radio receiving unit (RxRF) 31 performs power amplification, frequency conversion,
band filtering, analog-digital conversion, decoding and demodulation and other operations
on radio signals received from user apparatuses via an antenna and a duplexer. The
received radio signals may include downlink reception quality information, such as
a channel quality indicator (CQI) derived from a reception level for downlink reference
signals, and/or uplink reference signals.
[0021] The channel state evaluation unit 32 receives the channel quality indicator indicative
of downlink channel states (downlink CQI) from the user apparatuses or evaluates the
channel quality indicator for uplink channel states (uplink CQI) by evaluating a reception
quality SIR for the uplink reference signals received from the user apparatuses. The
channel state is determined based on one or both of the downlink CQI and the uplink
CQI. Strictly speaking, the channel state may be different for the uplinks and downlinks.
In this embodiment, averaged channel quality is used rather than instantaneous channel
quality for subsequent signal processing, and thus the channel state evaluation unit
32 may use one or both of the uplink CQI and the downlink CQI. This averaged channel
quality can be obtained through passage into a low pass filter, for example. The averaged
CQI cannot follow instantaneous fading but may change moderately to follow shadowing
and path loss (propagation loss).
[0022] The determination unit 33 determines whether a user apparatus resides in a cell boundary
based on the averaged CQI. If the averaged CQI is low, it is determined that the user
apparatus resides in the cell boundary. On the other hand, if the averaged CQI is
high, it is determined that the user apparatus resides near the base station.
[0023] The DRX group determination unit 34 determines which DRX group a user apparatus is
to belong to based on whether the user apparatus resides in a cell boundary and current
assignment status of the DRX groups. The current assignment status of the DRX groups
is stored in the assignment status memory 35.
[0024] FIG. 4 schematically illustrates exemplary contents stored in the assignment status
memory. In the illustration, "group number" corresponds to "pattern number" in FIG.
1. One or more user apparatuses belonging to a group identified by that number all
conduct discontinuous reception (DRX) while switching between the active state and
the inactive state at the same timings whereas user apparatuses belonging to different
groups are not activated at the same timing. The illustrated right column "total number
of UEs" indicates the total number of user apparatuses conducting the DRX at timings
for the associated group number. The illustrated center column "number of UEs in cell
boundary" indicates the number of user apparatuses residing in a cell boundary among
the user apparatuses conducting the DRX at timings for the associated group number.
In other words, the number of user apparatuses not residing in the cell boundary can
be derived from (total number of UEs) minus (number of UEs in cell boundary).
[0025] Referring back to FIG. 3, the DRX group determination unit 34 determines which group
a user apparatus of interest is to belong to based on the determination result of
the determination unit 33 and the current assignment status. In this embodiment, a
user apparatus residing in the cell boundary is assigned to a group having a smaller
total number of user apparatuses and a smaller number of user apparatuses residing
in the cell boundary. On the other hand, a user apparatus not determined to reside
in the cell boundary may be assigned to a group having a smaller total number of user
apparatuses.
[0026] The indicating signal generation unit 36 generates signals for indicating determined
DRX groups to user apparatuses. Although the DRX groups may be indicated in any appropriate
scheme, information indicative of the DRX groups may be included in data signals subsequent
to L1/L2 control signals due to infrequency reassignment of the DRX groups. More specifically,
the information indicative of the DRX groups may be indicated in MAC control PDUs
or L3-RLC messages.
[0027] The radio transmitting unit (TxRF) 37 transmits radio signals to user apparatuses
via a duplexer and an antenna. This transmission operation may include digital-analog
conversion, encoding, modulation, frequency conversion, band filtering and power amplification.
The radio signals may include L1/L2 control signals and data signals. The L1/L2 control
signals may include information indicative of user apparatuses, resource blocks and
transmission formats assigned for subsequent data signals, information indicative
of user apparatuses, resource blocks and transmission formats allowed for the next
uplink data transmission, and acknowledgement information (ACK/NACK) for previous
uplink data. The data signals may include signals indicative of the DRX groups together
with user traffic data destined for user apparatuses.
[0028] FIG. 5 is a flow chart illustrating an exemplary operation for a base station. At
step S1, the base station receives channel quality indicators indicative of downlink
channel state (downlink CQIs) from user apparatuses or evaluates channel quality indicators
indicative of uplink channel state (uplink CQIs) by evaluating receive quality SIRs
for uplink reference signals received from the user apparatuses. One or both of the
downlink CQIs and the uplink CQIs are averaged. It is determined whether the channel
state is satisfactory based on the averaged values. The averaging values may be obtained
through passage into a low pass filter, for example.
[0029] At step S2, it is determined based on the determined channel state whether a user
apparatus resides in cell boundary. If the averaged CQI for the user apparatus is
not satisfactory, the user apparatus is determined to reside in the cell boundary.
On the other hand, if the averaged CQI for the user apparatus is satisfactory, the
user apparatus is determined to reside near the base station.
[0030] At step S3, it is determined with reference to a current group assignment status
which DRX group the user apparatus is to belong to depending on whether the user apparatus
resides in the cell boundary. The respective DRX groups correspond to DRX patterns
corresponding to pattern numbers 1-10. All user apparatuses belonging to the same
DRX group switch between the active state and the inactive state in accordance with
the same DRX pattern. On the other hand, user apparatuses belonging to different DRX
groups are not activated at the same timing. Thus, the total number of user apparatuses
associated with a certain DRX group (group number or pattern number) may correspond
to the maximum number of user apparatuses that can be multiplexed in a downlink L1/L2
control signal transmitted in accordance with the corresponding pattern.
[0031] As stated above in conjunction with step S3, a user apparatus residing in the cell
boundary is assigned to s group having a smaller total number of user apparatuses
and a smaller number of user apparatuses residing in the cell boundary. A user apparatus
not determined to reside in the cell boundary may be assigned to a group having a
smaller total number of user apparatuses. For example, in the example illustrated
in FIG. 4, when the group assigned for the user apparatus residing in the cell boundary
is determined, group 3 or 4 may be a candidate for the assigned group.
[0032] At step S4, the DRX pattern determined at step S3 is indicated to the user apparatus,
and then the flow ends.
[0033] In the above-mentioned embodiment, ten types of DRX patterns have the same DRX cycle
and duty cycle. However, the present invention can be applied to multiple DRX patters
having different DRX cycles and/or different duty cycles. In addition, the present
invention can be applied to multiple patterns having partially matched activation
timings. In this case, for example, an averaged number of user apparatuses activating
simultaneously at each activation timing may be calculated for each pattern, and DRX
pattern assignments similar to the above-mentioned embodiment may be controlled based
on the calculated averaged number.
[0034] The present invention have been described with reference to the specific embodiments
of the present invention, but the embodiments are simply illustrative and variations,
modifications, alterations and substitutions could be contrived by those skilled in
the art. In the above description, some specific numerical values are used for better
understanding of the present invention. Unless specifically indicated, however, these
numerical values are simply illustrative and any other suitable values may be used.
For convenience of explanation, apparatuses according to the embodiments of the present
invention have been described with reference to functional block diagrams, but these
apparatuses may be implemented in hardware, software or combinations thereof. The
present invention is not limited to the above embodiments, and variations, modifications,
alterations and substitutions can be made by those skilled in the art without deviation
from the spirit of the present invention.
[0035] This international patent application is based on Japanese Priority Application No.
2007-075580 filed on March 22, 2007, the entire contents of which are hereby incorporated by reference.